Smoke detectors particularly ducted smoke detectors

ABSTRACT

The present invention relates to the detection of particles suspended in fluid particularly smoke detectors suitable for mounting on ducting for the early detection of smoke created by unwanted pyrolysis or combustion of materials in a protected area or fire zone to which the duct is connected. The present invention provides alternately illuminating a detection zone with one of either a first or a second illumination. The improvement embodied in the current invention is the ability to retain sensitivity to a wide range of particle sizes and also to discriminate between different kinds of smoke or dust according to particle size, whilst also achieving relatively long service life, small size, light weight and low cost.

CROSS REFERENCE TO RELATED APPLICATIONS

This is a U.S. National Phase Application under 35 U.S.C. § 371 ofInternational Application No. PCT/AU01/00121 filed Feb. 9, 2001, whichwas published Under PCT Article 21(2) in English, which claims priorityto Australian Application No. PQ 5538, filed Feb. 10, 2000, the entirecontents of which are incorporated herein by reference.

FIELD OF INVENTION

The present invention relates to the detection of particles suspended influid particularly smoke detectors. The invention is suitable formounting on ducting for the early detection of smoke created by unwantedpyrolysis or combustion of materials in a protected area or fire zone towhich the duct is connected.

The ducting may be ventilation or air-conditioning ducting used incontrolling the temperature and/or quality of air in the protected area.

The invention, equally may be free standing or provided in an openenvironment, that is, the invention does not require duct mounting. Theexamples disclosed are provided by way of explanation of the invention,only, and duct mounting is merely one preferred embodiment. The scope ofthe invention should not be so limited.

Duct mounted smoke detectors take a small sample of air passing throughan air duct such as a ventilator shaft and are intended to detect thepresence of smoke in the sample and thereby raise an alarm if theconcentration of smoke exceeds a predetermined value indicative of thepresence of a fire in the protected area.

Presently, conventional point type smoke detectors, primarily designedfor ceiling installation in a protected area, are used for ductedmounting. The detector is mounted inside a sealed housing to be mountedexternal to a duct, the housing is fitted with a pair of straighttubular probes which are fitted inside the duct and adapted to draw acontinuous small sample of air from the duct interior and to pass thesample or part thereof through the adjacent detector.

Difficulties arise if the smoke is significantly diluted as a result oflarge volumes of air passing through the duct. Faced with this dilution,it has been found that such detectors have insufficient sensitivity toprovide a warning that is appropriately early for life safety. Moreover,although a bug screen and dust filter is often included to protect thedetector from soiling, this is generally inadequate to prevent cloggingof passages or soiling of optical surfaces. Such detectors areinherently unreliable due to moisture condensation, soiling and falsealarms caused by dust, and are generally acknowledged to have anunsatisfactory service life measured only in months.

To overcome these disadvantages, high-sensitivity aspirated smokedetectors have been employed for duct monitoring. These detectorsprovide a sensitivity some hundreds of times greater than conventional,point detectors, thereby overcoming the smoke dilution.

The suction pressure available from the aspirator (air pump) canovercome the restriction of the dust filter, enabling a more efficientfilter to be employed thereby avoiding unwanted dust pollution andpossible false alarms.

Aspirated smoke detection has already been improved over many years bythe inventor as described in his Australian patent numbers 575845,3184384, 4229885, 3023684, 3184184, 3453784, 3400593, 8070891, 2774692,4050493 and 4050393, with corresponding patents overseas includingWestern Europe, North America, Japan and New Zealand.

An aspirated smoke detector employs an aspirator to draw a continuoussample of air through a dust filter and the smoke detection chamber.This aspirator may also draw samples of smoke from a ventilation duct,or alternatively through pipework for long distances.

In the case of pipework, this is of small bore and is often mounted on aceiling with sampling holes drilled at regular intervals, enablingsamples of air to be actively drawn from throughout the protected area.By contrast, conventional types of smoke detector rely upon convectioncurrents or air draughts to passively draw the smoke through thedetector chamber.

Whether intended for a duct or pipework application, ideally the smokedetector employed within an aspirated smoke detection system is anephelometer. This is a detector that is sensitive to all sizes of smokeparticles produced in fires, or during the early stages of overheating,pyrolysis or smoldering (which usually occurs for at least an hour priorto the appearance of flame).

Optical type smoke (or airborne particle) detectors of the prior arttypically use a single light source (or projector) to illuminate adetection zone that may contain such particles. A proportion of thislight may be scattered off the particles towards a single receiver cell(or sensor) that is positioned to provide acceptable detectionperformance. Improved versions of the prior art include one or moreadditional sensors positioned to receive light scattered in differentdirection(s). The output signals from these two or more sensors areutilized for the purpose of providing further information about theparticle size, or the average size of a group of particles. Adisadvantage of this prior art is that it uses a source of light havinga single wavelength, and is insensitive to the small particles producedin flaming fires.

Other detection techniques use a laser beam, providing a polarizedmonochromatic light source, typically in the near infrared wavelength.Such detectors are prone to having a high sensitivity to largeparticles, at the expense of having low sensitivity to small particles(that is, smaller than the wavelength of light). Thus a laser-baseddetector does not operate as a true nephelometer. This disadvantage canbe reduced by the use of a plurality of receiving sensors positioned todetect light scattered at various angles and polarizations, but only onelight wavelength is used.

Some aspirated smoke detectors have used a single laser diode beam butthis suffers the same disadvantage of using a single wavelength and haslow sensitivity to smoke from flaming fires. Other disadvantages ofusing aspirated detectors lie in terms of high cost, energy consumption,complexity and size.

The disadvantage exhibited by all of the above prior art whetheraspirated or not, namely their insensitivity to small particlescharacteristic of flaming fires, has in many instances demanded theinstallation of additional ionization type smoke detectors. Thesedetectors utilize a radioactive element such as Americium, to ionize theair within a chamber. The conductivity of this chamber is reduced whensmoke particles displace ionized air, causing an alarm to be operated.Such detectors are sensitive to the small particles produced in flamingfires but are insensitive to the large particles produced in pyrolysisor smoldering. These detectors are also prone to false alarms caused bydraughts which similarly displace ionized air. Accordingly theinsensitivity to incipient fires and the propensity toward false alarmsrenders ionization type detectors an unacceptable alternative.

Other, aspirated smoke detectors have used a Xenon lamp as the singlelight source that produces a continuous light spectrum similar tosunlight, embracing ultraviolet, visible and infrared wavelengths. Useof this continuous spectrum can detect particles of all sizes andproduce a signal that is proportional to the mass density of smoke,which is hitherto the most reliable measure of fire development.Although this can function as a true nephelometer, it does notcharacterize the type of fire. A disadvantage is the inability to selectparticular wavelengths unless a complex, costly and comparativelyunreliable system of mechanically moving color filters is used. Furtherdisadvantages of this technique are the service life of the Xenon lampwhich is typically limited to 4 years, the variation in light intensityand the costly high voltage power supply required.

Other prior art has used two light sources. GB 2193570 by Kane & Ludlow(May 10, 1980), for example, describes the use of one laser beam todetect the size and sphericity of airborne particles, requiring no lessthan five accurately positioned sensors. A second laser beam of the samewavelength is used to gate on and off the first laser, according to thepresence of a single particle in the field of view.

This second laser is used to improve the signal-to-noise ratio of thesystem, but not to determine the particle size or sphericity. Such asystem is too costly for the high-volume fire alarms industry.

As another example, U.S. Pat. No. 4,426,640 by Beconsall et al. (May 8,1986) describes a pollutant gas detector using two light sources butthis is not an airborne particle counter. This uses a first laseroperating at the absorption wavelength of the gas to be detected, and asecond laser operating at a reference wavelength which is necessarilysimilar, but not identical to the absorption wavelength. The two laserbeams are projected to “infinity” through the atmosphere (surrounding achemical plant) and the relative intensity of the signals received ateach wavelength provides a measure of the concentration of the pollutantgas.

It would be understood that the type of smoke produced in variouspyrolysis and combustion circumstances is different. Fast flaming firestend to produce a very large number of very small solid particles whichmay agglomerate into random shapes to form soot. In contrast, the earlystages of pyrolysis tend to produce a much smaller number of quite largeliquid particles (of high boiling point), typically existing as aerosolsthat may agglomerate to form larger, translucent spheres.

It has been found that the detection of large particles which slowlyincrease in quantity over an extended period of time would indicate apyrolysis or smoldering condition, requiring some attention.

Alternatively, the detection of numerous small particles arising quicklyand without an earlier pyrolysis or smoldering period, would tend toindicate arson where accelerants have been used and the need forimmediate action. An ability to distinguish between these extremes wouldassist the building operator, fire brigade or automatic fire alarmsystem in determining the appropriate response to the threat.

Another aspect of the prior art is its susceptibility to dust. Dust isimportant in two ways. Firstly, airborne dust is generally interpretedby the detector as smoke, so elevated dust levels can cause false firealarms. Secondly, even if discrimination means was used to reduce therate of false alarms, there remains the problem of soiling. Soiling isthe slow build-up of dust within the detector. This can affect thereliability of the detector by reducing its sensitivity to smoke and/orby reducing its safety margin against false alarms. The service life ofa detector is principally governed by soiling which consequentlyrequires regular maintenance. A detector that can minimize soiling andcan discriminate against smoke particles would be of advantage.Moreover, in certain applications the ability to identify the presenceof dust could be used to monitor the cleanliness of an area. Thisparticular role has hitherto required the use of very expensive dustparticle counters as used in the microchip fabrication industry whichare highly prone to soiling when applied to office type environments.

A smoke and/or dust detector that is rugged, of small size and oflightweight would be an advantage for applications in the aerospaceindustry.

OBJECTIVE OF THE INVENTION

It is an objective of the present invention to provide a smoke detectordevice having the ability to detect a wide range of particle sizes andto discriminate between different kinds of smoke or dust according toparticle size. The smoke detector device is suitable for mounting ontoan air conduit or ventilation duct. It is an objective to provide asmoke detector and a detection system having a relatively long servicelife with relatively long intervals between servicing.

It is an objective to provide a smoke detection system of relativelyhigh sensitivity capable of use without an aspirator.

STATEMENT OF INVENTION

There is provided according to a first aspect of the present invention adevice for the detection of particles suspended in a fluid, the deviceincluding light source(s) adapted to provide at least a first polarizedillumination and a second polarized illumination, a particle detectionzone through which a stream of sample fluid is adapted to flow, logicmeans adapted to alternately illuminate the detection zone with eitherthe first or second illumination, sensor means for reception of lightscattered off particles within the detection zone and output means toprovide an indication of a predetermined condition in the detectionzone.

Preferably, at least one of the polarized first and second illuminationis provided by light from the light source(s) being projected throughpolarizing filters, each with a different relative polarization.

Preferably, at least one of the polarized first and second illuminationis provided by a source of light having different polarization.

Preferably, the source of light having different polarization is a laserdiodes set to different polarization and/or wavelength.

Preferably, the above device has light source(s) which include at least2 light sources, the components of the device are mechanically fixed inposition, the first and second illuminations are independently radiated,the first and second illuminations are of different polarization, thefirst and second illuminations are provided from different positions,and/or the first and second illuminations are of different wavelengthsuch as one of short wavelength light and the other of long wavelengthlight.

There is provided according to a second aspect of the present inventiona device for the detection of particles suspended in a fluid, the deviceincluding a body portion, light source(s) adapted to provide at least afirst illumination and a second illumination, a particle detection zonethrough which a stream of sample fluid is adapted to flow, logic meansadapted to alternately illuminate the detection zone with either thefirst or second illumination, sensor means for reception of lightscattered off particles within the detection zone and output means toprovide an indication of a predetermined condition in the detectionzone, wherein the body portion is configured from two substantiallysimilar halves.

Preferably, the first and second illuminations are disposedsubstantially opposite an area of particle detection.

Preferably, the body portion is configured substantially axiallysimilar.

The above device(s) may or may not be duct mounted.

Preferably, the light source(s) includes a pair of light sources, one ofshort wavelength light the other of long wavelength light.

The improvement embodied in the current invention is the ability toretain sensitivity to a wide range of particle sizes and also todiscriminate between different kinds of smoke or dust according toparticle size, whilst also achieving relatively long service life, smallsize, light weight and low cost.

The light source(s) may be adapted to project light at the same anglerelative to the detection zone axis, or, alternatively, at a differentangle.

Typically the light source(s) is operated in a pulse mode such that onlyone wavelength is operated at one time. The system gain within theelectronic circuitry is adjusted so that under calibration conditions,each light source can produce the same signal level at the receivingsensor. In addition the receiving sensor is selected for its suitablebandwidth of operation (sensitivity to all of the wavelengths employed).

More than two wavelengths of light or polarized light or a combinationof the two may be utilized to achieve very high sensitivity to, ordiscrimination of, various types of particles encountered in thedetection chamber whether they be small or large smoke particles or dustparticles.

The receiving sensor may also have a polarizing filter. Operating none,or all of the light source(s) together at one time is also possible.

Thus, the light source(s) may be pulsed in sequence and both theabsolute and relative amplitude of pulses received at the sensor areanalyzed to determine the smoke concentration and the particle sizedistribution, thereby to characterize the smoke type.

According to a third aspect of the invention, there is provided a smokedetector and smoke detection method, in which the detector has a body,at least two light projectors mounted within the body for projectinglight into a detection zone adapted to receive an air sample, at leastone light receiving sensor mounted in the body to receive scatteredlight from the zone, the arrangement being such that the projected lightin pulses of differing wavelength, polarization and/or angle impingingupon the smoke and dust particles entering the detection zone willcreate scattered light indicative of a range of smoke particle sizesand/or the existence of dust particles, said sensor upon receiving atleast some of said scattered light being adapted to provide a signalwhich upon analysis enables the determination of smoke concentration andparticle size and/or size range.

According to a fourth aspect of the present invention, there is provideda method of smoke detection and a particle, smoke or dust detectorincluding a body having an inlet through which sample(s) of fluid,including air, can be provided, and output means for indicating an alarmcondition, the method and detector using a particle detection unithaving a source of light, and a particle size discrimination means,wherein the alarm condition is provided by analyzing over apredetermined period of time a change in the concentration of selectedparticle size(s) and/or range(s).

Preferably, the particle size(s) and/or particle range(s) determined arerelatively large particle size(s) and/or range(s).

Preferably, an alarm condition indicative of pyrolysis is provided upondetermining a relatively slow increase in large particle size(s) and/orrange(s).

Preferably, the particle size(s) and/or range(s) determined arerelatively small particle size(s) and/or range(s).

Preferably, an alarm condition indicative of a flaming fire is providedupon determining a relatively rapid increase in small particle size(s)and/or range(s).

Preferably, an alarm condition indicative of an accelerant being used isprovided upon determining that prior to the rapid increase in smallparticle size(s) and/or range(s), there was a small, if any, period ofpyrolysis.

Preferably, airborne dust content is determined in order to reduce falsealarms.

Preferably, separate alarm output is provided for any one of, or anycombination of, the alarm conditions noted above.

Preferably, the particle size discrimination means includes a firstlight source for detecting relatively small particle size(s) and/orrange(s) and a second light source for detecting relatively largeparticle size(s) and/or range(s).

Preferably, the first and second light source are alternatively active.

Preferably, the particle size discrimination unit utilizes a relativelyshort wavelength of light and a relatively long wavelength of light todetect particle size and/or range.

A still further aspect is directed to a smoke detector including thedetection unit and/or operatively adapted to detect an alarm conditionas disclosed herein.

A fifth aspect of the present invention provides an alarm detector andmethod of detecting an alarm condition for a pyrolysis, smolderingand/or smoke event, where a sample of fluid is provided, upon the fluidsample, impinging light emanating from a source of light, from theemanating light determining particle size(s), and over a predeterminedperiod of time, determining whether the number or concentration ofselected particle size(s) and/or range(s) has changed, in consequence ofwhich an alarm can be provided if the determination of concentration ofnumber of particles of selected particle size(s) and/or range(s) fallswithin selected criteria.

Preferably, the particle size(s) and/or particle range(s) determined arerelatively large particle size(s) and/or range(s).

Preferably, an alarm condition indicative of pyrolysis is provided upondetermining a relatively slow increase in large particle size(s) and/orrange(s).

Preferably, the particle size(s) and/or range(s) determined arerelatively small particle size(s) and/or range(s).

Preferably, an alarm condition indicative of a flaming fire is providedupon determining a relatively rapid increase in small particle size(s)and/or range(s).

Preferably, an alarm condition indicative of an accelerant being used isprovided upon determining that prior to the rapid increase in smallparticle size(s) and/or range(s), there was a small, if any, period ofpyrolysis.

Preferably, airborne dust content is determined in order to reduce falsealarms.

Preferably, separate alarm output is provided for any one of, or anycombination of, the alarm conditions noted above.

Preferably, in determining the particle size(s) and/or range(s), a firstlight source for detecting relatively small particle size(s) and/orrange(s) and a second light source for detecting relatively largeparticle size(s) and/or range(s) is used.

Preferably, in determining particle size(s) and/or range(s), the firstand second light source are alternatively active.

Preferably, in determining the particle size(s) and/or range(s), arelatively short wavelength of light and a relatively long wavelength oflight is used.

According to a further specific aspect of the present invention saidpulses of differing wavelength light may be of relatively shortwavelength such as violet or blue light and of relatively longwavelength such as red or infrared light.

According to a further specific aspect of the invention, the lightsource is generated by a light emitting diode (LED) having differingwavelength (colors) and/or utilizing a polarizing filter each filter setto a different relative polarization.

According to a further specific aspect of the invention, the lightsource is generated by a laser diode having differing wavelength(colors) and/or set to a different relative polarization.

There is also provided according to the invention in a smoke detectorsystem including at least one smoke detector the improvement includingsampling of fluid from within a duct and transmitted to at least onedetector as described above.

There is also provided according to the invention in a structure havingducting the improvement wherein fluid is sampled for the detection of apre-determined condition from the duct.

In one specific aspect of the invention sample air from the duct isdrawn through a probe mounted within the duct containing an inlet andoutlet port.

In a sixth aspect of the invention the sample air may be drawn directlyfrom the duct or tube into the smoke detector device wherein the duct ortube is formed with a venturi construction to generate sufficientrelative pressure between the detector chamber and the duct.

In essence, one aspect of the present invention comes about havingrealized that more than one wavelength of light is required to detect amore complete range of particle sizes and types of fire, and todiscriminate among them. Another aspect of the present invention hasimportantly found that determining particle concentration, size and/orrange(s) over a period of time can give a very good indication ofwhether an alarm condition has been met or is warranted. Yet anotheraspect of the present invention stems from having at least two sourcesof light illuminating a particle detection zone and a detection meansproviding an output signal indicative of a predetermined condition ofthe particle detection zone. Having two sources of light enablesparticle size discrimination to be achieved while using no more than onereceiving sensor. Yet a further aspect of the present invention is therecognition of dust particles for the monitoring of dust levels or forthe avoidance of false fire alarms.

DESCRIPTION OF THE DRAWINGS

FIG. 1 a is a sectional plan view taken on line 1—1 of a smoke detectorbody;

FIG. 1 b illustrates in plan view, an alternative smoke detector body;

FIG. 1 c illustrates in plan view, a further alternative smoke detectorbody;

FIG. 2 is a sectional elevational view taken on line 2—2 of the smokedetector body;

FIG. 3 is a cross-sectional view taken on line 3—3 of a smoke detectorbody showing the gas sample inlet pipework;

FIG. 4 is a cross-sectional view taken on line 4—4 of a smoke detectorbody showing its filter chamber and diffuser ducting;

FIG. 5 is a sectional view taken on line 5—5 of the smoke detector bodyand housing;

FIG. 6 is a sectional view taken on line 6—6 of the smoke detector bodyand housing;

FIG. 7 is an end view of the inlet/outlet gas port to the smoke detectorbody with gasketing;

FIG. 8 is a sectional side view of duct probe taken on a line C—C;

FIG. 8 a is an end view of the probe that attaches to the smoke detectorbody;

FIG. 8 b is a cross-sectional view of the probe taken on a line E—E;

FIG. 8 c is an end view of the probe remote from the detector body;

FIG. 9 is a side elevational view of the duct sampling probe;

FIG. 10 is a sectional view of an alternate duct or pipe samplingconfiguration;

FIGS. 11 a and 11 b show side views of an alternative high volume probe;

FIG. 12 a shows a section view of an alternative probe, with thedetector body attachment removed;

FIG. 12 f shows a high volume detector body attachment;

FIG. 12 k shows a low volume detector body attachment;

FIGS. 12 b–12 e and 12 g–12 j show various views of the probe of FIG. 12a; and

FIGS. 13 a and 13 b show side views of an alternative low volume probe.

DESCRIPTION OF PREFERRED EMBODIMENTS

In general terms, the present invention seeks to detect airborneparticles and/or to provide discrimination according to particle sizeusing apparatus that has low cost, small size, low weight, highruggedness, high reliability, low maintenance and long service life, andis suitable for high production volumes. This is achieved with the useof only a single sensor, together with at least two inexpensive lightsources. Use of a single sensor and its associated electronic amplifiernecessarily designed for high sensitivity with low noise, simplifies thedesign and reduces the cost of the system. It also avoids any lack ofconsistency that could occur in the sensitivity and linearity ofadditional sensors and it avoids the possibility of the incrementaladdition of noise contributions from plural sensors.

Discrimination of airborne particle size could be achieved in a numberof ways. The two or more light sources may differ in wavelength,polarization, position (specifically the solid angle of incidence to thedetection zone axis), or a combination of these.

In the preferred embodiment of the invention, two light emitting diodes(LED's) operating at different wavelengths are employed. This permitsthe use of wavelengths as distant as 430 nm (blue) and 880 nm (infrared)such that the wavelengths are separated by a full octave. Such a largedifference in wavelength can produce a significantly different strengthof signal when light of alternate wavelength is scattered off particlestoward the sensor. Alternative combinations such as 430 nm (blue) with660 nm (red) are possible. Closer-spaced wavelengths such as 525 nm(green) with 660 nm (red) could be used, accompanied by a reduction insize discrimination and sensitivity to small particles.

It is known from Rayleigh theory that the intensity of the scatteredlight reduces according to the fourth power of wavelength, for particlessmaller than the wavelength of light. This has proven relevant to smokedetection in experiments using Xenon lamps which produce a completespectrum embracing infrared, visible and ultraviolet wavelengths, whereit was found that wavelengths in the blue region are necessary for thedetection of certain kinds of fires liberating small particles.

Therefore, a particular advantage of being able to employ a blue lightsource is that its short wavelength provides high resolution of smallparticles that become invisible at longer wavelengths. Whereas a blue orviolet laser diode may be preferable to a blue LED, the former areexpensive, have increased alignment complexity, require automatic powercontrol and have a lower tolerance of elevated temperatures. Thecombination of readily available red and infrared laser diodes could beused, but in addition to the difficulties presented by using lasers,these longer wavelengths fail to adequately resolve small particles.

Accordingly the preferred embodiment of the invention is configured toutilize the broad beam spread of a high-intensity LED (approx 12 deg).Although the broad spread of the LED beam could be confined by focusingwith a lens, this adds cost, complexity in alignment and size to theproduct. Whereas the LED does not have the localized high lightintensity of a collimated laser beam, the aggregate intensity of the LEDlight scattered from the large volume of the detection zone whenintegrated on the sensor is of comparable magnitude. Therefore thesensitivity of the LED based system is comparable with laser, but thecost is reduced without compromising reliability.

Nevertheless, the same invention could be configured to use laser diodesas alternative light sources of differing wavelength, polarization orposition (angle). Such arrangements can provide particle sizediscrimination also, but at a higher cost and greater temperatureintolerance than LED designs.

The ability to use LED's is achieved by the novel configuration of theoptical chamber which accommodates the broad projector beam angle ofeach LED, opposite a specially designed light trap located beyond thedetection zone, to completely absorb the remnant projected light,thereby preventing its detection at the sensor. The chamber alsocontains a further light trap opposite the sensor and beyond thedetection zone, to eliminate stray projected light from being detected.Thus the signal-to-noise ratio caused by remnant projected lightcompared with the detected scattered light, is maximized to ensure veryhigh sensitivity of the system. This is further ensured by the closemutual proximity of the LED's and the sensor to the detection zone, sothat inverse-square light intensity losses are minimized. Moreover, alens is preferably used in conjunction with the sensor to gatherscattered light from throughout the detection zone while minimizingvisibility of chamber wall surfaces as a result of focusing. Controlirises are used to further minimize stray light reaching the sensor.Through the combination of all these methods the system sensitivity ison the order of 0.01 to 0.1%/m equivalent smoke obscuration.

It should be noted that the ability to utilize a broad projector beamenables the use of laser diodes without costly collimation optics.

In one preferred embodiment of the invention, each light source ispulsed in sequence for a short period such as 10 mS. At the sensor, asignal is generated in response to each pulse of scattered light at eachwavelength. The system is pre-calibrated to account for the sensitivityof the sensor at each wavelength, preferably by adjusting the intensityof the LED projections during manufacture. The signals are amplifiedusing digital filtering to improve the signal-to noise ratio, and boththe absolute and relative amplitudes of the pulse signals are stored.The absolute value indicates the particle concentration whereas therelative value indicates the particle size or the average size of agroup of particles. From Rayleigh theory, at a given mass concentrationof airborne particles, the long wavelength light will produce a lowamplitude signal in the case of small particles, or a large amplitudesignal in the case of large particles. The short wavelength light willproduce a relatively equal amplitude signal in the case of both smalland large particles. By comparing the ratio of the signals it istherefore possible to determine whether the particles are large orsmall.

Signals produced over a period of time are analyzed according to trend.A slow increase in the concentration of large particles is indicative ofpyrolysis and eventually a smoldering condition. Alternatively, a rapidincrease in small particles is indicative of a fast flaming fire and, inthe absence of a prior period of pyrolysis and smoldering, couldindicate the involvement of accelerants (such as with arson). Thisinformation is used to produce separate alarm outputs in the case ofsmoldering and flaming fires, or alternatively, to reduce the alarmactivation threshold (i.e., provide earlier warning) in the case offlaming fires (which are more dangerous).

It should be noted that the concentration of smoke alone, does notnecessarily indicate the level of danger of an incipient fire. Theconcentration detected will depend upon the degree of smoke dilution byfresh air, and the proximity of the incipient fire to the detector. Bycharacterizing the smoke in accordance with our invention it becomespossible to determine the level of smoke concentration necessary for analarm, that is appropriate to the protected environment, therebyproviding early warning with minimum false alarms. Moreover, the lowcost of the system encourages its comprehensive use throughout afacility.

In a further embodiment of the invention, particle size discriminationis used to determine the airborne dust content for the purpose ofavoiding false alarms or for dust level monitoring within the protectedenvironment. Two LED's may be used, but by the use of additional LED'sit is possible to discriminate within differing particle size ranges.

Preferred embodiments of the present invention will now be describedwith reference to the accompanying drawings.

In one embodiment of the invention, and referring to FIG. 1, the smokedetector housing 10 is produced by the molding of two substantiallyidentical halves 10 a, 10 b (see FIG. 4). Two LED lamps 11 arepositioned to project light across the detection chamber 12 into aregion that is viewed by the sensor 13. Smoke 14 is drawn across thechamber 12 in the direction of arrows 15 so that it can be irradiated bythe projectors 11 in sequence. Some light 16 scattered off the airbornesmoke particles is captured by a focusing lens 17 onto the receivingsensor 13.

A series of optical irises 18 confine the spread of the projector beamsand another series of irises 19 confine the field of view of the sensor13. An absorber gallery 39/40 (light trap) is provided opposite eachprojector 11 to absorb essentially all of the remaining essentiallyunscattered light and thereby prevent any swamping of the scatteredlight 16 at the sensor 13 by the projected light. A further light trap20 is provided opposite the sensor to further ensure that essentially noprojector light is able to impinge on the sensor.

The smoke detector housing 10 preferably incorporates pipework 21 toprovide airflow through the detector chamber 12. This pipework 21 mayincorporate a nozzle 22 opposite a collector 23, to direct the airflowacross the chamber 12, such that the chamber is quickly purged of smokein the event that the smoke level is reducing. Included in the pipeworkpathway is a dust filter 33. Coupling to the dust filter cavity is byinlet and outlet diffusers 24, 25 designed to minimize head loss(pressure drop) in the airflow through the detector, and to facilitatethe use of a large filter 33 for long service life. Over a period ofyears, a small quantity of fine dust may pass through the filter. Toprevent or minimize soiling, the arrangement of the nozzle and collectoris such as to minimize deposition of dust on the chamber walls andoptical surfaces.

FIGS. 1 b and 1 c illustrate alternative positioning of the lightsource(s) 11 of FIG. 1 a. This has necessitated the re-positioning ofthe light trap 39, 40. In many other respects, the features of FIGS. 1 band 1 c are identical to the illustration of FIG. 1 and the accompanyingdescription. FIGS. 1 b and 1 c do not show all the detail of FIG. 1 a,only as a matter of clarity. It is to be noted that FIGS. 1 b and 1 callow for backscatter detection or a combination of back and forwardscatter, i.e., different angles.

FIG. 2 illustrates a sectional elevation view taken along line 2—2 ofthe smoke detector body of FIG. 1. Again, many features shown in FIG. 1a are numbered identically. FIG. 2 indicates the preferred position ofthe main electronics printed circuit board PCB1 for efficient andlow-interference electrical connection to the projecting light sourcesand the receiving sensor including its pre-amplifier printed circuitboard PCB2. Conveniently the upper half of the smoke detector body 10 bmay be removed without disturbing the connections to PCB1 for thepurposes of setup and maintenance.

Referring to FIG. 3, there is shown a cross-sectional view taken alongline 3—3 of FIG. 1 and showing the gas sample inlet pipework includingsocket and bends.

A cross-sectional view taken on line 4—4 of FIG. 1 shows its filterchamber and is represented in FIG. 4. The filter element is preferablyof open-cell foam construction with a relatively large filter pore sizesuch as 0.1 mm. This causes dust particles to be arrested progressivelythroughout the large depth of the element. Use of such a large pore sizemeans that smoke particles are not arrested in the filter, even when thefilter becomes loaded with dust, which if it occurred would reduce thesensitivity of the detector to smoke. This element is easily removed forcleaning or renewal.

In FIG. 5, there is a sectional view taken long line 5—5 of the smokedetector body of FIG. 6. This indicates how the detector body and thedetector housing are secured with screws, and in exploded view showswhere the housing may be attached to the duct such as a circularventilation duct (which is more challenging than a flat-sided duct). Forexample, attachment may be achieved by screws, magnets or adhesive tape.

FIG. 6 illustrates a sectional view taken on line 6—6 of FIG. 5 of thesmoke detector body. FIG. 1 a also shows line 6—6. In FIG. 6 a view ofthe outer casing, mounted on a pcb PCB1, together with a gasket 31 isshown. This particular arrangement is suitable for mounting to a duct,although the present invention should not be limited to only such anapplication.

FIG. 7 is an end view of the inlet/outlet gas port to the smoke detectorbody showing gasket 31 in plan view. This gasket provides a releasableseal to a duct such as a round ventilation duct of unspecified radius

The following description relates to one preferred arrangement of theinvention, and with reference to FIGS. 8, 8 a, 8 b, 8 c and 9. It is tobe noted that the following description equally applies to thealternative high volume and low volume embodiments shown in FIGS. 11 a,11 b, 12 a to 12 k, and 13 a and 13 b. The same numeral references havebeen used in the various figures to avoid duplication. The high volumeembodiment is used when fluid flow in the duct is relatively high. Thusthe inlet and outlet openings 28 and 29, respectively are designed to besmaller, so with a high volume of fluid flow, a smaller sample area iscaptured and substantially the same volume of fluid to the detector ofthe present invention. Equally, the low volume embodiment is designedwith relatively larger openings 28 and 29, as the fluid flow is lower, alarger opening is provided to present substantially the same amount offluid flow to the detector of the present invention.

The pipework is configured with appropriate bends and sockets suitablefor attachment to a probe 26, which draws smoke from the ventilationduct 27. The probe 26 is preferably of unit construction containing aninlet port 28 and an outlet port 29, so that only one penetration hole30 need be cut into the duct wall to provide access for the probe 26.This hole is releasably sealed using a closed-cell foam gasket 31 toprevent leakage. FIG. 8 shows a view along line C—C from FIG. 8 b. FIG.12 a also shows a view along line C—C of FIGS. 12 c and 12 h. FIG. 8 ashows a view along line D—D of FIG. 8. FIG. 12 b shows a view along lineD—D of FIG. 12 a for the high volume embodiment. FIG. 12 g shows a viewalong line D—D of FIG. 12 a for the low volume embodiment. FIG. 8 bshows a sectional view along line E—E of FIG. 8 indicating that itcomprises a stem with a detachable head. FIGS. 12 c and 12 h show,respectively, high volume and low volume embodiments of the probe viewedalong line E—E of FIG. 12 a. FIG. 8 c shows a view along line F—F. FIGS.12 e and 12 j show plan views of the, respective, high volume and lowvolume probes. FIGS. 12 d and 121 show sectional views of the heads ofthe, respective high and low volume probes.

The probe 26 is suitable for being inserted into a duct by requiringonly a single round penetration of the duct. The probe is inserted sothat its inlet faces upstream and its outlet faces downstream. The probeis designed to provide an adequate airflow rate through the detectionchamber 12, driven by the dynamic head associated with the airflow inthe ventilation duct 27. This dynamic head produces a pressure dropacross the inlet port 28 and outlet port 29 of the probe 26, sufficientto overcome the combined restriction of the detection chamber 12,pipework 21 and dust filter 33. The efficiency of the probe is maximizedby the use of rounding of the inlet orifice followed by a bend to changethe direction of the sampled flow with minimum loss. This is repeated atthe outlet. The inlet and outlet bends are incorporated without anyrequirement to enlarge the duct penetration. This high efficiencyenables the use of an effective dust filter to ensure a long serviceinterval for the product, such as 10 years in a typical officeenvironment. Given such a long interval, it is considered appropriate(but not essential) that the detector body 10 can be easily dismantledfor cleaning and re-calibration, avoiding the need for a removablefilter cartridge that is costly and difficult to make airtight. The highefficiency of the probe also facilitates its use in ventilation ductsoperating at relatively low air velocity such as 4 m/sec. For use at lowventilation duct velocities, an alternative probe head is provided. Thisuses an enlarged air scoop design which incorporates a diffuser toefficiently accelerate the inlet air and ensure that the detector'srapid response to smoke is maintained.

In a preferred embodiment of the invention, with reference to FIGS. 8 band 9, the probe 26 is constructed with an elliptical or similarcross-section that will minimize drag (to minimize restriction to flowin the ventilation duct), as well as minimizing forced vibration at theStrouhal frequency caused by the duct flow. In the particular embodimentillustrated by FIGS. 8 b and 9, the aerodynamic coefficient of drag isreduced by a factor of ten compared with a pair of round pipes ofsimilar dimensions. FIGS. 12 b to 12 k show similar features, but inrespect of the high and low volume probes. The advantages of using anelliptical shape instead of an aerofoil are that the probe may beinstalled in either direction, and that the overall width of the probeis reduced, without unduly compromising the reduction in drag. By theaddition of further stem sections, the probe 26 may be extended inlength to meet the needs of different sized ductwork, ensuring adequateflow without the need of an aspirator. The pressure inside the duct 27can be significantly different from the ambient atmosphere outside theduct (where the detector is usually mounted). In a preferred embodimentof the invention best shown in FIGS. 1 and 6, the halves of the chamberare releasably joined in an airtight manner by means of only onecontinuous O-ring seal 34. This sets the detector chamber internalpressure to approximate that of the ventilation duct and avoids anyleakage to or from ambient atmosphere.

Leakage into the detector could cause an unwanted alarm from smoke inthe ambient environment. Leakage of smoke from the detector to theambient environment could cause an unwanted alarm in other smokedetection equipment protecting that environment.

Alternatively, with reference to FIG. 10 if a relatively small duct orpipe is used such that the probe in inappropriate, then this duct may beconfigured to produce a venturi which develops the necessary pressuredrop to ensure an adequate flow rate through the detector chamber,filter and pipework. Again only a small proportion of the smoke need bepassed through the detector and this proportion is minimized in order tominimize the rate of detector soiling and filter loading, thereby tomaximize the service interval.

1. A device for detection of particles suspended in a fluid, the devicecomprising light source(s) adapted to provide at least a first polarizedillumination and a second polarized illumination, a particle detectionzone through which a stream of sample fluid is adapted to flow, logicmeans adapted to alternately illuminate the detection zone with eitherthe first or second illumination, sensor for reception of lightscattered off particles within the detection zone and output means toprovide an indication of a predetermined condition in the detectionzone, wherein a majority of light received by the sensor is the lightscattered off particles within the detection zone.
 2. The device asclaimed in claim 1, wherein at least one of the polarized first andsecond illuminations is provided by light from the light sources(s)being projected through polarizing filters, each with a differentrelative polarization.
 3. The device as claimed in claim 1 wherein atleast one of the polarized first and second illuminations is provided bya source of light having different polarization.
 4. The device asclaimed in claim 3, wherein the source of light having differentpolarization is a laser diode set to a different polarization and/orwavelength.
 5. The particle detection device as claim in claim 1,wherein the light sources(s) comprises at least two light sources, thelight sources(s), the particle detection zone, the logic means, thesensor and the output means are mechanically fixed in position, thefirst and second illuminations are independently radiated, the first andsecond illuminations are of different polarization, the first and secondilluminations are provided from different positions, and/or the firstand second illuminations are of different wavelength, one of shortwavelength light and the other of long wavelength light.
 6. A smokedetector comprising the particle detection device as claimed in claim 1.7. A probe adapted to be installed in a conduit, such as a duct throughwhich a fluid may flow, the probe being adapted to communicate fluidlywith a device, detector or detection unit as claimed in claim 1, theprobe comprising a first portion adapted to be inserted into theconduit, the first portion having an inlet for receiving an upstreamfluid flow, and an outlet portion having an outlet for outputting adownstream flow, the inlet and outlet portions each having a channelwhich is curved to enable a change of direction of the fluid flow withinthe probe while substantially minimizing flow resistance.
 8. A devicefor detection of particles suspended in a fluid, the device comprising abody portion, light source(s) adapted to provide at least a firstillumination and a second illumination, a particle detection zonethrough which a stream of sample fluid is adapted to flow, logic meansadapted to alternately illuminate the detection zone with either thefirst or second illumination, sensor for reception of light scatteredoff particles within the detection zone and output means to provide anindication of a predetermined condition in the detection zone, whereinthe body portion is configured from two substantially similar halves,and a majority of light received by the sensor is the light scatteredoff particles within the detection zone.
 9. The device as claimed inclaim 8, wherein the first and second illuminations are disposedsubstantially opposite to an area of particle detection.
 10. The deviceas claimed in claim 9, wherein the body portion is configuredsubstantially axially similar.
 11. A smoke detector comprising theparticle detection device as claimed in claim
 8. 12. A particle detectorhaving a body, at least two light projectors mounted within the body forprojecting light into a detection zone adapted to receive an air sample,at least one light receiving sensor mounted in the body to receivescattered light from the detection zone, the light projectors beingadapted to project light into the detection zone, wherein the projectedlight includes pulses of differing wavelength, polarization and/or angleimpinging upon smoke and dust particles entering the detection zone andcreating scattered light indicative of a range of smoke particle sizesand/or the existence of dust particles, said sensor upon receiving atleast some of said scattered light being adapted to provide a signalwhich upon analysis enables the determination of particle concentrationand particle size and/or range, wherein a majority of light received bythe sensor is scattered light created by the projected light impingingupon smoke and dust particles entering the detection zone.
 13. A smokedetector comprising the particle detector as claimed in claim
 12. 14.The device as claimed in claim 1, wherein dust particles entering thedetection zone scatter light indicative of one of a range of smokeparticle sizes, the existence of dust particles, and a range of smokeparticle sizes and the existence of dust particles, said sensor meansupon receiving at least some of said scattered light providing a signalenabling the determination of particle concentration and particle sizeand/or range.
 15. The device as claimed in claim 8, wherein dustparticles entering the detection zone scatter light indicative of one ofa range of smoke particle sizes, the existence of dust particles, and arange of smoke particle sizes and the existence of dust particles, saidsensor means upon receiving at least some of said scattered lightproviding a signal enabling the determination of particle concentrationand particle size and/or range.
 16. A device for detecting particlescomprising: at least two light sources for projecting light of differingwavelength, polarization and/or angle; and at least one sensor forreceiving scattered light, wherein the scattered light is produced bythe projected light impinging upon particles, said sensor providing asignal representing at least one of particle concentration, particlesize and particle range, wherein a majority of light received by thesensor is the scattered light produced by the projected light impingingupon the particles.
 17. A method of detecting particles comprising:projecting light of differing wavelength, polarization and/or angle fromat least two light sources; receiving scattered light produced by theprojected light from the at least two light sources impinging uponparticles by at least one sensor, wherein a majority of light receivedby the sensor is the scattered light produced by the projected lightfrom the at least two light sources impinging upon particles; andgenerating a signal representing at least one of particle concentration,particle size and particle range based on the received scattered light.18. The device as claimed in claim 1, wherein substantially all of thelight received by the sensor is the light scattered off particles withinthe detection zone.
 19. The device as claimed in claim 8, whereinsubstantially all of the light received by the sensor is the lightscattered off particles within the detection zone.
 20. The particledetector as claimed in claim 12, wherein substantially all of the lightreceived by the sensor is scattered light created by the projected lightimpinging upon smoke and dust particles entering the detection zone. 21.The device as claimed in claim 16, wherein substantially all of thelight received by the sensor is the scattered light produced by theprojected light impinging upon the particles.
 22. The method as claimedin claim 17, wherein substantially all of the light received by thesensor is the scattered light produced by the projected light from theat least two light sources impinging upon particles.